Various rechargeable battery systems exist for energy storage based on the reversible storage of hydrogen. These systems include nickel-metal hydride, nickel-hydrogen, silver-metal hydride, silver-hydrogen and hydride-manganese dioxide. Further, various metallic alloys are known to reversibly store hydrogen in electrochemical batteries. Materials having this capability include nickel-titanium, iron-titanium, lanthanum-nickel, mischmetal-nickel, cerium-nickel, magnesium-copper, samarium-cobalt, and a variety of other substituted alloys.
Current state of the art methods for preparing working electrodes from hydrogen storage alloys consist of the combination of sintering and the use of binders, or binders alone, to hold the hydrogen storage alloy electrode materials together. In order to produce an electrode, the hydrogen storage alloy must be processed into a finely divided powder with a small particle size which is typically less than 100 mesh, prior to any electrode processing. The powder particles are essentially spherical in geometry. Close-packing spherical particles allows minimal contact between the particles, resulting in poor conductivity and reduced structural integrity. Further, this method of manufacture has certain additional disadvantages. Hydrogen storage alloys are extremely hard and brittle and are not conducive to the preparation of finely divided powders. Further, the powder preparation step involves the use of hydrogen which embrittles the powder. Subsequent high speed grinding adds processing steps and therefore additional costs to the material. After preparation of the finely divided powder, this material can be readily oxidized, corroded, and may become passivated in the presence of atmospheric oxygen and moisture. As a result, satisfactory material handling in a battery manufacturing operation presently requires complicated processing steps.
Also, the use of binders in the electrode has disadvantages. Binders in a sintered electrode design are selected to burn off in the high temperature sintering operation. However, no material burns without leaving some residue. The binder residue left in the electrode structure affects the electrode and battery, affects performance, and provides micro-sites in the electrode which have an increased susceptibility to corrosion when contacted by the battery electrolyte. Binders used in non-sintered electrodes remain in the electrode throughout the electrode's operational life. These binders are non-conductive materials, for example, polytetrafluoroethylene, polyethylene, methyl cellulose, latex and other polymers and plastics. These materials are known to increase the impedance of the electrode and decrease the wetting ability of the electrode. Both effects are detrimental to electrode performance.
In the manufacture of an electrochemical battery, the alloys of the type discussed above are formed into the negative electrode. As hydrogen is absorbed into the metallic lattice of the negative electrode during charging, the structure necessarily expands in three-dimensional degrees of freedom resulting in volume expansion of both the electrode material and structure. This change in volume is a characteristic of the alloy and can be as much as 20%. As the metallic lattice expands, metal-to-metal bonds within the electrode structure are broken, resulting in decreased physical contact between alloy particles, decreasing both conductivity of the electrode structure and mechanical strength.
Hydrogen uptake and release, known also as the hydriding and dehydriding process, corresponds to the charging and discharging of the electrodes in the battery as the volume of the alloy changes. During this process, the metal particle-to-particle bonds are stress-fractured due to the brittle nature and inherent hardness of the hydride alloys. This problem also extends to the sintered electrodes, which have enhanced rigidity due to both the sintered matrix and the physical, fused bonds between the metal particles. The expansion and contraction of both sintered and nonsintered electrodes during the hydriding/dehydriding process not only initially decreases the physical contact between the alloy particles with a resulting decreased conductivity, but ultimately causes the electrodes to fail due to physical deterioration and subsequent disintegration.